A helicopter commonly has a main lift and propulsion rotor that is provided with a plurality of blades. The blades of the main rotor describe a very open cone having its plane of rotation perpendicular to the general lift generated by said main rotor. This general lift of the main rotor can then be resolved into a vertical lift force and a horizontal force that drives the helicopter in translation. Consequently, the main rotor does indeed provide lift and propulsion for a helicopter. Furthermore, by controlling the shape and the angle of inclination of said cone relative to the frame of reference of the helicopter, a pilot can direct the helicopter accurately.
In order to act on the cone, the blades are caused to flap so as to modify their angle of inclination relative to the drive plane of the main rotor, which drive plane is perpendicular to the mast of the main rotor. Consequently, the helicopter is provided with specific means for the purpose of varying the pitch of each blade, and consequently the aerodynamic angle of incidence of each blade relative to the incident stream of air through which the blade passes.
By varying the pitch of a blade, the lift it generates is varied and that has the consequence of causing the blade to flap. In order to control the general lift of the main rotor, both in magnitude and in direction, the pilot generally acts on the value of the pitch angle of each blade by causing the blade to turn about its longitudinal pitch axis. Thus, when the pilot orders collective variation of pitch, i.e. pitch variation that is identical for all of the blades, the pilot causes the magnitude of the general lift from the main rotor to vary so as to control the altitude and the speed of the helicopter. In contrast, collective pitch variation has no effect on the direction of the general lift.
In order to modify the direction of the general lift generated by the main rotor, it is appropriate to tilt said cone by varying pitch not collectively but cyclically. Under such circumstances, the pitch of a blade varies as a function of its azimuth angle, and during one complete revolution it passes through a maximum value and a minimum value that occur at opposite azimuth angles.
Cyclic variation of blade pitch gives rise to cyclic variation of the lift from the blades, and thus varies the tilt of the cone. By controlling cyclic variation of blade pitch, the pilot controls the attitude of the aircraft and its movement in translation. The flight controls enabling the pilot to control pitch, a collective pitch lever and a cyclic stick, are generally connected to three servo-controls via a mechanical connection referred to as “linkage” that is connected to the non-rotary plate of a cyclic swashplate. Furthermore, the rotary plate of the cyclic swashplate is mechanically connected to each of the blades by a pitch control rod.
When the pilot seeks to modify the collective pitch of the blades, action on the collective pitch lever instructs the three servo-controls to raise or lower the cyclic swashplate as a whole, i.e. both the rotary and the non-rotary plates thereof. The pitch control rods are thus all moved through the same distance, which implies that the pitch of all of the blades is varied by the same angle.
In contrast, in order to vary the cyclic pitch of the blades so as to direct the helicopter in a given direction, the pilot tilts the cyclic stick appropriately so as to cause at least one of the servo-controls to move. The cyclic plate does not move vertically but instead tilts relative to the mast of the main rotor. Each pitch control rod is thus moved, thereby generating pitch variation for each blade.
The mechanical connection is also provided with at least one connecting rod and with at least one crank means for connecting the pilot's flight controls of the servo-controls. This mechanical connection is also provided both with a mixer unit that enables the cyclic stick and the collective pitch lever to act independently of each other, and also with a phasing unit that enables the cyclic swashplate to tilt about two perpendicular axes in heavy helicopters. This mechanical connection is thus in general very long and heavy. Under such conditions, the pilot can have difficulty in moving the cyclic stick or the collective pitch lever, given the force that needs to be applied, particularly if the helicopter is itself heavy.
A first solution consists in using electric flight controls as suggested in documents WO 2005/002963 or US 2007/0102588. Nevertheless, that first solution is difficult to implement, particularly on existing rotorcraft. Consequently, manufacturers have remedied the problem posed by adding a hydraulic or pneumatic power assistance system. Known systems providing power assistance consist in a block of actuators acting dynamically merely as reducing cranks, the block of actuators being arranged between the bottom crank means and the mixer unit, for example.
Nevertheless, those power-assistance systems are bulky and heavy. In addition, they run the risk of leaking, hydraulically or pneumatically, thereby leading to a loss of effectiveness. Finally, the gas in pneumatic power-assistance systems is sensitive to variations in temperature, where such variations are unfortunately frequent in aviation, while the fluid used in hydraulic power-assistance systems contains polluting chemicals.